It is possible to image particular molecules, cells or organisms using optical imaging techniques. Bioluminescence imaging provides a relatively inexpensive means of imaging genetically-engineered luciferase-expressing cells. When luciferase-labelled cells or genes are exposed to substrate (e.g., luciferin, injected just prior to imaging), photons are emitted that can be detected and quantified by a charge-coupled device (CCD) camera. Relatively small numbers of cells (< 1,000) can be detected with this technique because background noise levels are inherently low. The availability of genetically-altered luciferases and natural luciferase variants, characterized by differing emission spectra, may allow simultaneous tracking of more than one molecular target.Using fluorescently labelled antibodies or other molecules, or inducing expression of fluorescence proteins, fluorescence imaging can also be used for cellular and molecular imaging. Unlike bioluminescence imaging, the animal is typically illuminated with an external light source. The resulting autofluorescence reduces the signal-to-noise ratio, but background filtration can be used to improve sensitivity. Diverse fluorescent dyes are now commercially available, allowing imaging of several fluorophores simultaneously in a single animal. In addition, dual-function reporter genes, e.g, a luciferase-GFP fusion reporter, have been utilized to couple in vivo imaging with ex vivo assays, and triple-fusion genes have been developed to link optical and PET imaging.In vivo optical imaging is limited by the absorption and scatter of light as it passes through living tissue. The development of optical probes that transmit in the near infra-red spectrum has improved resolution, but optical imaging will likely remain a lower resolution modality in comparison to PET, SPECT, CT and MRI. Nevertheless, it has widespread and important application in many fields of biological and biomedical research, including tumor growth and response to therapy, efficacy of gene transfer/gene expression, bacterial infection and immune cell trafficking, beta-amyloid plaque development, and drug efficacy.